hammer2 - Add 'snapshot' directive

hammer2 - Add 'snapshot' directive

* Add the snapshot directive, which by default snapshots the current
  directory or the specified directory.

  A label is optional.  If not specified the PFS name prefix and a timecode
  postfix is constructed.  Snapshots are created in the super-root, so use
  pfs-list to list them along with any discrete PFS's.

* Fix the pfs-delete directive.

* Subdirectories of the H2 mount can be snapshotted with the caveat that
  hardlinks which cross above the snapshot directory will not be in the
  snapshot (have lookup failures when accessed via the snapshot).